1. Field of the Invention
The present invention relates to a chemically processed steel sheet having a converted layer, which is excellent in workability and corrosion resistance at both a flat plane and a worked or machined part, generated on a surface of an Alxe2x80x94Si alloy plating layer.
2. Description of Related Art
Al-coated steel sheets have been used as steel material excellent in the corrosion-resistance. But, when the Al-coated steel sheet is held as such in a humid atmosphere, exhaust gas or an environment subjected to dispersion of sea salt grains for a long time, its external appearance is worsened due to generation of white rust on the Al plating layer. Chromating effectively inhibits generation of white rust on a surface of the Al-coated steel sheet from the following reasons.
A chromate layer generated on a surface of a steel base is composed of complex oxides and hydroxides of trivalent and hexavalent Cr. Scarcely-soluble compounds of Cr(III) such as Cr2O3 acts as a barrier against a corrosive atmosphere and protects a steel base from corroding reaction. Compounds of Cr(VI) are dissolved as oxoatic anions such as Cr2O72xe2x88x92from the converted layer and re-precipitated as scarcely-soluble compounds of Cr(III) due to reducing reaction with exposed parts of a steel base formed by working or machining. Re-precipitation of Cr(III) compounds autogenously repairs defective parts of the converted layer, so that a corrosion-preventing effect of the converted layer is still maintained after working or machining.
Although chromating is effective for corrosion prevention of a steel sheet, it obliges a big load on post-treatment of Cr ion-containing waste fluid. In this regard, chemical liquors containing compounds such as titanium compounds, zirconium compounds or phosphates have been developed for generation of converted layers (hereinafter referred to as xe2x80x9cCr-free layers), which do not contain chromium compounds or Cr ion, and some are already applied to aluminum DI (drawn and ironed) cans. For instance, JP 9-20984 Al proposed an aqueous solution containing titanium compound, sulfuric phosphate, fluorides and an accelerator for coating an Al-containing metal part with a chemically converted (titanium compound) layer.
Titanium compound, zirconium compound or phosphate-containing converted layers, which have been proposed instead of the conventional chromate layer, do not exhibit such a self-repairing faculty as the chromate layer. For instance, a titanium compound layer does not exhibit a self-repairing faculty due to insolubility, although it is uniformly generated on a surface of a steel base in the same way as the chromate layer. As a result, the titanium compound layer is ineffective for suppression of corrosion starting at defective parts formed during chemical conversion or plastic deformation of a steel sheet. The other Cr-free layers are also insufficient for corrosion prevention due to poor self-repairing faculty.
When a small amount of a Cr-free chemical liquor is spread on an Al-coated steel sheet by a conventional method using an applicator roll or a spray wringer, an Al plating layer is not uniformly coated with a converted layer. The un-coated parts, i.e. surface parts where the Al plating layer is exposed to an atmosphere, act as starting points for corrosion or scratching during working, resulting in occurrence of damages in the converted layer or the Al plating layer. When a relatively thick converted layer is generated so as to completely cover the plating layer by spreading an excessive amount of a Cr-free chemical liquor on the contrary, defects such as cracks easily occur in the converted layer during press-working, since the converted layer cannot follow to deformation of a steel base. The defects in addition to an insufficient self-repairing faculty cause degradation of corrosion-resistance.
The present invention aims at provision of a chemically processed steel sheet remarkably improved in corrosion resistance by generating a converted layer, which contains both soluble and scarcely-soluble metal compounds, with a self-repairing faculty on an Alxe2x80x94Si alloy plating layer formed on a steel base.
The present invention proposes a new chemically processed steel sheet having a steel base coated with an Alxe2x80x94Si alloy plating layer containing 5-13 mass % Si. A surface of the plating layer is preferably reformed to a rugged state by concentration of Si so as to distribute Si-rich particles as convex parts thereon. Such distribution of Si-rich particles is attained concentration of Si to 7-80 mass % at a surface of the plating layer.
A converted layer, which is generated on the rugged surface, contains a complex compound of Ti and Mn. The complex compound may be one or more of oxides, hydroxides, fluorides and organic acid salts. The converted layer may further contain one or more of phosphates, complex phosphates and lubricants. Concentration of Si at a surface of the plating layer is preferably controlled under the condition such that Si content within a range from the surface to at least 100 nm depth is adjusted to 7-80 mass %.
Another converted layer, which contains one or more oxides or hydroxides of valve metals together with fluorides, is also effective for corrosion prevention. The valve metal has the feature that its oxide exhibits high insulation resistance. The valve metal is selected from Ti, Zr, Hf, V, Nb, Ta, Mo and W. The self-repairing faculty of the converted layer is typically noted by addition of one or more fluorides to the converted layer at an F/O atomic ratio not less than 1/100. The converted layer optionally contains organic or inorganic lubricants.
The converted layer may further contain one or more of soluble or scarcely-soluble metal phosphates or complex phosphates. The soluble metal phosphate or complex phosphate may be a salt of alkali metal, alkaline earth metal or Mn. The scarcely-soluble metal phosphate or complex phosphate may be a salt of Al, Ti, Zr, Hf or Zn.
Manganese compounds and valve metal fluorides are effective components other than chromium compound, which give a self-repairing faculty to a converted layer, since these compounds are dissolved in water and then re-precipitated as scarcely-soluble compounds at defective parts of the converted layer.
The manganese compound in the converted layer is partially changed to a soluble component with a self-repairing faculty. Accounting the self-repairing faculty of the manganese compound, the inventors experimentally added various kinds of chemical agents to a liquor for generation of a converted layer containing the manganese compound, and researched effects of the chemical agents on corrosion-resistance of the converted layer. In the course of the researches, the inventors discovered that addition of a titanium compound to the chemical liquor is effective for suppressing dissolution of the converted layer and for bestowing the converted layer with a self-repairing faculty, as disclosed in JP Application No. 3302677B.
The titanium compound improves stability and corrosion-resistance of a converted layer containing a manganese compound. On the basis of such the effect of the titanium compound, the inventors have further researched for a method which can inhibit exposure of an Al plating layer through a converted layer generated even at a relatively small ratio, and discovered that a substrate suitable for improvement of corrosion-resistance is an Alxe2x80x94Si alloy-coated steel sheet with concentration of Si at a surface of a plating layer. It is assumed that increase of Si content in at surface improves corrosion-resistance of the converted layer from the following reason:
When an Alxe2x80x94Si alloy-coated steel sheet having Si concentrated at its surface is held in contact with a chemical liquor, Al is selectively etched away from the surface of the Alxe2x80x94Si plating layer, so that the surface of the plating layer is reformed to a rugged state having convex parts composed of metallic Si and concave parts enriched with Al. Since the chemical liquor is easily gathered in the concave parts, the concave parts are preferentially coated with complex compounds of Ti and Mn. The Si-rich convex parts and the Al-rich concave parts may be formed by acid-pickling, alkali-degreasing or the like in prior to the chemical converting.
When the converted layer is generated in this way, the surface of the Alxe2x80x94Si plating layer is reformed to a hard rugged state due to presence of metallic Si and a complex compound of Ti and Mn. The rugged surface favorably reduces an area (in other words, friction resistance) of the plating layer held in contact with a metal die during press-working. Such the state that Al-rich parts are scarcely exposed on the surface of the plating layer is also effective for anti-scratching property and reduction of Al picked up to an electrode during resistance-welding, resulting in a long life time of the electrode. Furthermore, when a paint is applied to the converted plating layer, adhesiveness of a paint film is improved due to an anchoring effect of the rugged surface. Even if defects such as cracks occur in the converted layer which cannot follow to plastic deformation of a steel base during press-working or machining, the defects are eliminated by the self-repairing faculty of the manganese compound. Consequently, good corrosion-resistance is still maintained even at the worked or machined part.
The self-repairing faculty is also realized by presence of a valve metal fluoride in a converted layer. In this case, a valve metal oxide or hydroxide is incorporated together with the fluoride in the converted layer. The valve metal is an element, whose oxide exhibits high insulation resistance, such as Ti, Zr, Hf, V, Nb, Ta, Mo and W. The converted layer acts as a resistance against transfer of electrons due to inclusion of the valve metal oxide(s) or hydroxide(s) and suppresses reducing reaction caused by oxygen dissolved in water (oxidizing reaction of a steel base, in turn). Consequently, dissolution (corrosion) of metal components from a steel base is inhibited. Especially, tetravalent compounds of Group-IV A metals such as Ti, Zr and Hf are stable components for generation of converted layers excellent in corrosion resistance.
The oxide or hydroxide of the valve metal is effective as a resistance against transfer of electrons, when a converted layer is uniformly generated on a surface of a steel base. However, occurrence of defective parts in a converted layer is practically unavoidable during chemical conversion, press-working or machining. At the defective parts where the steel base is exposed to an atmosphere, the converted layer does not sufficiently inhibit corroding reaction. A soluble valve metal fluoride incorporated in the converted layer effectively realizes a self-repairing faculty for corrosion-prevention at the defective parts. The valve metal fluoride is once dissolved to water in an atmosphere and then re-precipitated as an scarcely-soluble oxide or hydroxide on a surface part of the steel base exposed through defective parts of the converted layer. Re-precipitation of the valve metal oxide or hydroxide repairs the defective parts, and the faculty of the converted layer for corrosion prevention is recovered.
For instance, a titanium compound layer generated on a surface of a steel base is composed of TiO2 and Ti(OH)2. When the titanium compound layer is microscopically observed, defects such as pinholes and very thin parts are detected in the titanium compound layer. The defects act as starting points for corroding reaction, since the steel base is exposed to an atmosphere through the defects. Although a conventional chromate layer exhibits a self-repairing faculty due to re-precipitation of a scarcely-soluble Cr(III) compound at defective parts, such the self-repairing faculty is not expected as for the titanium compound layer. Defective parts of the converted layer are reduced by thickening the converted layer, but the hard titanium compound layer poor of ductility does not follow to plastic deformation of a steel base during working the chemically processed steel sheet. As a result, defects such as cracks and biting easily occur in the converted layer during working or machining.
On the other hand, co-presence of a fluoride such as XnTiF6 (X is an alkali metal, an alkaline earth metal or NH4, and n is 1 or 2) or TiF4 in the converted layer promotes dissolution of a fluoride to water in an atmosphere and re-precipitation of a scarcely-soluble oxide or hydroxide according to the formula of TiF62xe2x88x92+4H2Oxe2x86x92Ti(OH)4+6Fxe2x88x92. The re-precipitation means realization of a self-repairing faculty. A metal part of the fluoride may be either the same as or different from a metal part of the oxide or hydroxide. Some oxoates of Mo or W useful as a valve metal exhibit such the self-repairing faculty due to solubility, so as to relax restrictions on a kind of a fluoride to be incorporated in a converted layer.
The above-mentioned control of Si content in an Alxe2x80x94Si alloy plating layer also effectively inhibits exposure of Al in case of the titanium compound layer by the same reasons. The converted layer is uniformly generated on a rugged surface of an Alxe2x80x94Si alloy plating layer, and exposure of Al-rich parts is inhibited by controlling Si content of the plating layer. Defects such as cracks would occur in the converted layer during press-working, since the converted layer does not follow to plastic deformation of a steel base. Such the defects are eliminated by the self-repairing faculty of the converted layer, so that the steel sheet still maintains sufficient corrosion resistance even at the deformed part.
A steel base may be low-C, medium-C, high-C or alloyed steel. Especially, low-C Ti- or Nb-alloyed steel is suitable as a steel base which will be deeply drawn to an objective shape at a heavy working ratio.
The steel base is coated with an Al plating layer by a conventional hot-dip process. The plating layer preferably contains 5-13 mass % Si. Si content not less than 5 mass % favorably accelerates concentration of Si at a surface of the plating layer and also inhibits growth of an alloyed layer, which puts harmful influences on workability, at boundaries between the steel base and the plating layer. However, excessive Si content more than 13 mass % promotes precipitation of primary Si in the plating layer during cooling succession to hot-dipping and significantly degrades workability of the coated steel sheet.
After a steel sheet coated with an Alxe2x80x94Si alloy plating layer whose Si content is controlled in a range of 5-13 mass % is raised from a hot-dip bath, it is cooled at a controlled cooling speed so as to concentrate Si at a surface of the plating layer. Thereafter, the coated steel sheet is pickled with an acid or degreased with an alkali, so that its surface is reformed to a rugged state comprising Si-rich convex parts and Al-rich concave parts. In this case, the coated steel sheet is washed with water and then dried. The rugged surface may be formed by treating the hot-dip coated steel sheet with a chemical liquor, which has etching activity on Al, instead of acid-pickling or alkali-degreasing. In this case, Al is selectively etched off a surface of the plating layer at a time when the steel sheet is dried to generate a converted layer thereon after application of the chemical liquor. Due to selective removal of Al from the plating layer, the surface of the plating layer is reformed to a rugged state.
The situation that Si-rich convex parts and Al-rich concave parts are distributed on a surface of a plating layer is confirmed by AES analysis for scanning and analyzing an area of 1 mmxc3x971 mm and an Ar sputtering method for repeatedly analyzing the plating layer in a region from the surface to 100 nm depth. Results of experiments prove that concentration of Si not less than 7 mass % in the region from the surface to 100 nm depth effectively improves corrosion-resistance at both a flat plane and a worked or machined part. However, if Al is excessively etched off the plating layer until Si content exceeds 80 mass %, the surface of the plating layer becomes so fragile that a converted layer generated thereon would be easily peeled off without following to deformation of a steel sheet during press-working.
A complex compound layer containing one or more of manganese compounds for realization of a self-repairing faculty is generated by applying an aqueous solution containing titanium and manganese compounds to a hot-dip coated steel sheet, and then drying the steel sheet as such. The titanium compound may be one or more of K2TiF6, TiOSO4, (NH4)2TiF6, K2[TiO(COO)2], TiCl4, Ti(SO4)2 and Ti(OH)4. The manganese compound may be one or more of Mn(H2PO4)2, MnCO3, Mn(NO3)2, Mn(OH)2, MnSO4, MnCl2 and Mn(C2H3O2)2.
The chemical liquor preferably contains a manganese compound at a ratio of 0.1-100 g/l calculated as Mn. Concentration of Mn not less than 0.1 g/l is necessary for deposition of manganese compound effective for improvement of corrosion-resistance, but excessive concentration of Mn more than 100 g/l unfavorably degrades stability of the chemical converting liquor. A titanium compound is preferably added to the chemical liquor at such the ratio that a mole ratio of Ti/Mn is controlled in a range of 0.05-2. A Ti/Mn mole ratio not less than 0.05 assures improvement of corrosion-resistance without degrading a self-repairing faculty of the converted layer. An effect of the titanium compound on improvement of corrosion-resistance is noted at a Ti/Mn mole ratio more than 2, but an excessive Ti/Mn mole ratio causes instability of the chemical liquor and raises a processing cost.
An organic acid with chelating faculty may be further added to the chemical liquor, in order to maintain scarcely-soluble metals (e.g. Ti and Mn) as stable metal ions in the chemical liquor. Such the organic acid may be one or more of tartaric, tannic, citric, malonic, lactic and acetic acids. The organic acid is preferably added to the chemical liquor at an organic acid/Mn mole ratio of 0.05-1. An effect of the organic acid on stability of the chemical liquor is noted at an organic acid/Mn mole ratio not less than 0.05, but an organic acid/Mn mole ratio more than 1 causes falling of a pH value of the chemical liquor and degradation of continuous processability.
The chemical liquor is adjusted at a pH value in a range of 1-6 by quantitatively controlled addition of a titanium compound, a manganese compound, phosphoric acid or a phosphate, a fluoride and an organic acid at proper ratios. A pH value below 1 accelerates dissolution of Al and worsens continuous processability, but a pH value above 6 causes precipitation of titanium compounds and instability of the chemical liquor.
A converted layer containing valve metal fluoride(s) for realization of a self-repairing faculty is generated by spreading either a coat-type or reaction-type chemical liquor to an Alxe2x80x94Si alloy-coated steel sheet. The reaction-type chemical liquor is preferably adjusted to a relatively low pH value to assure its stability. In the following explanation, Ti is used as a valve metal. The other valve metals are also used in the same way.
A chemical liquor contains a soluble halide or oxoate as a Ti source. Titanium fluoride is useful as both Ti and F sources, but a soluble fluoride such as (NH4)F may be supplementarily added to the chemical liquor. In concrete, the Ti source may be XnTiF6 (X is an alkali or alkaline earth metal, n is 1 or 2), K2[TiO(COO)2], (NH4)2TiF6, TiCl4, TiOSO4, Ti (SO4)2 or Ti (OH)4. Ratios of these fluorides are determined such that a converted layer having predetermined composition of oxide(s) or hydroxide(s) and fluoride(s) is generated by drying and baking a steel sheet to which the chemical liquor has been spread.
An organic acid with chelating faculty may be further added to the chemical liquor, in order to maintain a Ti source as a stable ion in the chemical liquor. Such the organic acid may be one or more of tartaric, tannic, citric, oxalic, malonic, lactic and acetic acids. Especially, oxycarboxylic acids such as tartaric acid and polyhydric phenols such as tannic are advantageous in stability of the chemical liquor, assistance to a self-repairing faculty of a fluoride and adhesiveness of a paint film. The organic acid is preferably added to the chemical liquor at an organic acid/Mn mole ratio not less than 0.02.
An F/O atomic ratio of a converted layer is preferably adjusted to a value not less than {fraction (1/100)} in order to realize a self-repairing faculty of a fluoride in the converted layer. F and O atoms in the converted layer are analyzed by X-ray fluorescence, ESCA or the like. The self-repairing faculty derived from hydrolysis of a fluoride is insufficient at an FIO atomic ratio less than {fraction (1/100)}, so that defective parts of the converted layer or cracks formed in the converted layer during press-working sometimes act as starting points for propagation of corrosion.
Orthophosphates or polyphosphates of various metals may be added for incorporation of soluble or scarcely-soluble metal phosphates or complex phosphates in a converted layer.
A soluble metal phosphate or complex phosphate is dissolved from a converted layer, reacted with Al in a plating layer through defective parts of the converted layer and re-precipitated as a scarcely-soluble phosphate which assists a self-repairing faculty of manganese oxide or hydroxide or titanium fluoride. An atmosphere is slightly acidified on dissociation of the soluble phosphate, so as to accelerate hydrolysis of manganese oxide or hydroxide or titanium fluoride, in other words generation of scarcely-soluble compounds.
A metal component capable of generating a soluble phosphate or complex phosphate is an alkali metal, an alkaline earth metal, Mn and so on. These metals are added as metal phosphates alone or together with phosphoric acid, polyphosphoric acid or phosphate to the chemical liquor.
A converted layer containing manganese compound(s) for realization of a self-repairing faculty is further improved in corrosion-resistance by addition of phosphoric acid or phosphate as a component for generation of a scarcely-soluble phosphate to a chemical liquor. The phosphate may be manganese phosphate, sodium dihydrogenphosphate, disodium hydrogenphosphate, magnesium phosphate and dihydrogenammonium phosphate. The phosphoric acid or phosphate is preferably added to the chemical liquor at a P/Mn mole ratio not less than 0.2 for improvement of corrosion-resistance. However, a P/Mn mole ratio more than 4 causes instability of the chemical liquor.
A scarcely-soluble metal phosphate or complex phosphate may be dispersed in a converted layer containing a fluoride for realization of a self-repairing faculty, so as to eliminate occurrence of defects and to improve strength of the converted layer. A metal component capable of generating a scarcely-soluble phosphate or complex phosphate is Al, Ti, Zr, Hf, Zn and so on. These metals are added as metal phosphates alone or together with phosphoric acid, polyphosphoric acid or phosphate to the chemical liquor.
Such a fluoride as KF, NaF or NH4F, which is easily dissociated to fluoride ion as an etching element to Al, may be added to the chemical liquor. These fluorides may be added alone or together with a fluoride with small dissociation constant such as silicofluoride or with titanium or manganese fluoride. The fluoride is preferably added to the chemical liquor at a F/Mn mole ratio not more than 10.
The prepared chemical liquor is spread to an Alxe2x80x94Si alloy-coated steel sheet by an applicator roll, a spinner, a sprayer or the like, and then the steel sheet is dried as such without washing. Consequently, a converted layer good of corrosion-resistance is generated on a surface of the plating layer. The chemical liquor is preferably applied to the plating layer at a ratio not less than 1 mg/m2 calculated as deposited Mn or Ti for realization of excellent corrosion-resistance. A quantitative effect of the chemical liquor on corrosion-resistance is saturated at a ratio of 1000 mg/m2 calculated as deposited Mn or Ti, and further improvement of corrosion-resistance is not expected even if the chemical liquor is applied at a ratio more than 1000 mg/m2 for generation of a thicker converted layer.
The steel sheet, which has a converted layer generated from the chemical liquor applied to a surface of a plating layer, may be dried at an ordinary temperature, but preferably dried within a short time at a temperature of 50xc2x0 C. or higher accounting continuous processability. However, drying at a too-higher temperature above 200xc2x0 C. causes thermal decomposition of organisms in case of generating a converted layer containing organisms, resulting in degradation of corrosion-resistance.
The converted layer can be bestowed with lubricity by addition of a lubricant to a chemical liquor, in order to suppress occurrence of damages in the converted layer as well as the plating layer during press-working or machining. The lubricant may be one or more of powdery synthetic resins, for instance polyolefin resin such as fluorocarbon polymer, polyethylene, and polypropylene, styrene resin such as ABS and polystyrene or halide resin such as vinyl chloride and vinylidene chloride. Inorganic powder such as silica, molybdenum disulfide, graphite or tungsten disulfide is also used as a lubricant. An effect of the lubricant on workability of a chemically processed steel sheet is noted at a ratio of the lubricant to the converted layer being not less than 1 mass %. Excessive addition of the lubricant at a ratio more than 25 mass % impedes generation of the converted layer and worsens corrosion-resistance.
An organic paint film good of corrosion resistance may be laid on the converted layer. Such the paint film is formed by applying a resin paint containing one or more of olefinic resins such as urethane, epoxy, polyethylene, polypropylene and ethylene-acrylic copolymer, styrenic resins such as polystyrene, polyesters, acrylic resins or these copolymers or degenerated resins. The resin paint may be applied to the converted layer by an applicator roll or electrostatic atomization. When a paint film of 0.5-5 xcexcm in thickness is laid on the converted layer, the converted layer surpasses a conventional chromate layer in corrosion resistance.
Lubricity during press-working is ensured by addition of an organic or inorganic lubricant to the paint film. Resistance-weldability is improved by addition of inorganic sol. The paint film may be either alkali-soluble or insoluble. Alkali-solubility of the paint film is controlled by a ratio of acrylic acid incorporated in the resin. The paint film becomes alkali-soluble as increase of the acrylic acid, and insoluble as decrease of the acrylic acid.